This invention relates to an apparatus for drying particulate material comprising at least one particulate material container which is connected in a closed air circulating system with an air drying device and a heater, in which a dry heated gas is moved through the particulate material by means of a blower in order to extract moisture from the particulate material, and air discharged from the particulate material container subsequently is dried again, reheated by means of a heater and then supplied again to the particulate material container, wherein a connection for introducing dry, heated gas is provided in an upper region of the particulate material container, the connection leading into a duct arranged substantially centrally in the container and having a lower end provided with a gas distributor.
An apparatus of this type is disclosed published German Utility Model No. DE-GM 89 10 764.2. This known apparatus supplies dried air via a laterally flanged-on duct to the container for the particulate material; the dried air being heated by an electric heater and flowing into the particulate material to be dried in the lower area of the particulate material container. With this apparatus, the particulate material is heated in the immediate vicinity of the container outlet. The warm dry air flows upwards, heats the particulate material and extracts the moisture from it, and the moisture is discharged through an exhaust air outlet in the upper region of the container.
As the result of the arrangement of the heater on the side of the container for the particulate material and the introduction of heated air there into the container, a temperature difference exists in this area, on the one hand, with respect to the outgoing air and, on the other hand, with respect to the particulate material. In addition, the flow of the particulate material in this area is impaired by the arrangement of the heater and the air supply duct.
U.S. Pat. No. 4,858,335 (=DE 3,637,700) discloses another apparatus for drying particulate material, in which dried heated air is supplied to the center of a container for the particulate material and flows out at the lower end of the container via an air distributing duct.
A disadvantage of this known apparatus is that the air supply duct is surrounded by particulate material and the particulate material directly adjacent the air supply duct is strongly heated and under certain circumstances may suffer thermal damage.
While, in the apparatus disclosed in German Utility Model No. DE-GM 89 10 764.2, the air supply duct, which is mounted laterally on the container, has a disadvantageous effect on the flow of product through the apparatus, the air supply duct in the apparatus disclosed in U.S. Pat. No. 4,858,335, which extends from the top center along the whole cylindrical height of the container, makes it possible to achieve a uniform mass flow of particulate material through the apparatus, but this latter construction has the disadvantage that it is difficult to control the inlet temperature of the air introduced into the granulate.
Thus, for example, in a high-temperature drying system in which the heater is flanged directly to the container and the temperature sensor for the heater control is situated directly behind the heater, the inlet temperature of the drying air introduced into the granulate is lower than the temperature measured by the temperature sensor because considerable radiation losses occur on the downward path through the cold granulates or the cold return air and the drying air will therefore cool down.
If the temperature sensor is arranged directly in the lower portion of the air distributor, this means that the air temperature in the upper portion of the air supply duct is clearly higher and the granules, which are in direct contact with the duct, may be subjected to an excessively high temperature.